1. Field of the Invention
The present invention relates to an optical pickup actuator that can be driven centering on the same driving axis as an optical axis of an objective lens, and improves sensitivity by preventing subsidiary resonance caused by a leakage magnetic flux, and an optical pickup, and an optical recording and/or reproducing apparatus using the same.
2. Description of the Related Art
Typically, an optical pickup is a device that is used in an optical recording and/or reproducing apparatus, moves in a radial direction of an optical disc as an optical recording medium, and performs recording and/or reproducing of information on and/or from the optical disc in a noncontact manner.
The optical pickup requires an optical pickup actuator, which drives an objective lens in tracking and focusing directions of an optical disc, so that light emitted from a light source is formed as a light spot in an appropriate position on an optical disc. But the desire to make a portable personal computer (PC), such as a notebook, thin and light, causes limitations in available space in such a computer. Thus, an actuator used in the portable PC should be made slim.
A reflector is used in the optical pickup to direct light toward the objective lens. As the actuator should be made slim, an asymmetric actuator having a driving axis different from an optical axis of the objective lens has been proposed, to reduce a distance between the objective lens and the reflector of the optical pickup. An example thereof is disclosed in U.S. Pat. No. 5,684,645.
Referring to FIGS. 1 and 2, a holder 14 is positioned at one side of a conventional optical pickup actuator 10, a focusing coil 18 is wound along an outer circumference of a lens holder 12, a first accommodation groove 16a is positioned at a center of the lens holder 12, and a pair of tracking coils 15 are wound at one side of the lens holder 12. In addition, a second accommodation groove 16b is positioned in a moving portion 17, on which an objective lens 11 is mounted, and the lens holder 12 is accommodated in the second accommodation groove 16b. Here, a U-shaped yoke 31 is inserted into the first and second accommodation grooves 16a and 16b, and a magnet 32 is mounted at one side of the U-shaped yoke 31 to face the pair of tracking coils 15.
One end of each of a pair of suspensions 13a and 13b is fixed in the holder 14, and the respective other ends thereof are fixed at sides of the moving portion 17. The moving portion 17 is supported by the pair of suspensions 13a and 13b, and the moving portion 17 and the lens holder 12 are combined with each other to move together.
If a current is applied to the focusing coil 18 and the tracking coil 15, a force is applied to the coils 18 and 15 by an electromagnetic interaction between the magnet 32 and the focusing coil 18 or the tracking coil 15, such that the moving portion 17 moves. A direction in which the force is applied to the focusing coil 18 and the tracking coil 15 follows Flemings Left Hand Rule.
Thus, if an electromagnetic force acts on the coils 18 and 15 by the electromagnetic interaction between the magnet 32 and the focusing coil 18 or the tracking coil 15, the lens holder 12 moves in a focusing direction F or a tracking direction T. As such, the moving portion 17 combined with the lens holder 12 moves together, and simultaneously, the objective lens 11 moves, and a position in which a light spot is formed in a disc (not shown) is adjusted.
FIGS. 3A and 3B schematically show an electromagnetic interaction between the focusing coil 18 and the magnet 32. Here, the focusing coil 18 includes a portion 18a placed inside the U-shaped yoke 31 and a portion 18b placed outside the U-shaped yoke 31. However, an electromagnetic force is applied to the portion 18a of the focusing coil 18 placed inside the yoke 31 by an interaction between the portion 18a of the focusing coil 18 and the magnet 32. On the other hand, the portion 18b of the focusing coil 18 placed outside the yoke 31 is blocked by the yoke 31 and thus, is not affected by the magnet 32. But in actuality, as indicated by a dotted line of FIG. 3A, a magnetic flux generated in the magnet 32 is deviated from the center of the magnet 32 and is widely spread at an edge of the magnet 32. Then, the magnetic flux is deviated from the yoke 31 and is leaked to the outside.
The portion 18b of the focusing coil 18, placed outside the yoke 31, is affected by a leakage magnetic flux. Arrows from the focusing coil 18 of FIG. 3A represent the size and direction of a force applied to the focusing coil 18 by distribution of a magnetic flux in accordance with Flemings Left Hand Rule. In this way, a force is applied to the portion 18b of the focusing coil 18 placed outside the yoke 31 by a leakage magnetic flux, and this causes distribution of a nonuniform force applied to the focusing coil 18. In other words, as is shown in FIG. 3B, a force Fu applied to the portion 18a of the focusing coil 18 placed inside the yoke 31 and a force Fd applied to the portion 18b of the focusing coil 18 placed outside the yoke 31 are nonuniform such that a pitching mode, in which the lens holder 12 and the moving portion 17 are shaken back and forth, is formed. That is, the lens holder 12 and the moving portion 17 are shaken in a direction of arrow P of FIG. 3B.
In addition, the portion 18b of the focusing coil 18 placed outside the yoke 31 is a coil that is not used for a focusing operation, and due to an increase in mass and wound coil resistance, sensitivity of the actuator is lowered. Thus, a problem occurs in a high-speed follow capability caused by high-speed of a disc.
Since a motion centroid H of the tracking coils 15 is not identical with a mass centroid G of the tracking coils 15 during motion in the tracking direction T caused by the pair of tracking coils 15, a rolling mode is formed. As is shown in FIG. 4A, when the lens holder 12 stops, a mass centroid G of the actuator 10 is identical with a motion centroid H of the lens holder 12. Arrows of FIG. 4A represent the size and direction of a force applied to the tracking coils 15 by the magnet 32. The size of a force applied to the tracking coils 15 depends on the size of current and a magnetic flux flowing through the tracking coils 15. When a current is constant, the size of the force applied to the tracking coils 15 depends on only the size of a magnetic flux. The magnetic flux is the largest in the center of the magnet 32, and is gradually reduced closer to an edge of the magnet 32.
As is shown in FIG. 4A, when the tracking coils 15 are in a neutral position, a magnetic flux is distributed symmetrically with respect to the tracking coils 15. As such, the mass centroid G of the tracking coils 15 is identical with the motion centroid H of the tracking coils 15.
But as is shown in FIG. 4B, if the lens holder 12 is focused upward by the focusing coil 18, a force applied to the tracking coils 15 by the magnet 32 is deflected to a lower portion of the tracking coils 15. Thus, since a tracking force in a downward direction of the lens holder 12 is larger than a tracking force in an upward direction of the lens holder 12, a rotational moment occurs in a direction of arrow R1.
On the other hand, as is shown in FIG. 4C, if the lens holder 12 is focused downward by the focusing coil 18, a force applied to the tracking coils 15 by the magnet 32 is deflected to an upper portion of the tracking coils 15. Thus, since a tracking force in the upward direction of the lens holder 12 is larger than a tracking force in the downward direction of the lens holder 12, a rotational moment occurs in a direction of arrow R2.
Consequently, as is shown in FIG. 4D, since the motion centroid H and the mass centroid G of the tracking coils 15 are not identical with each other according to a focusing direction of the lens holder 12, a rolling mode that rolls in directions of arrows R1 and R2 is formed.
A rotational vibration mode, such as a pitching mode and a rolling mode, affects the phase and displacement of basic frequency characteristics during focusing and tracking operations, and thus, an optical signal is decreased. Thus, when the size of the magnet 32 is made large to increase magnetic flux density for improvement of AC sensitivity, a leakage magnetic flux is increased, subsidiary resonance occurs, and there is a limitation in an increase in magnetic flux density. Further, in a high-speed and high-density optical recording and/or reproducing apparatus, the pitching mode and the rolling mode are often generated. Thus, a high speed optical pickup actuator suitable for an optical recording and/or reproducing apparatus is needed.
In the case of tilting driving, in addition to focusing driving and tracking driving, the number of suspensions is at least six. It is very difficult to perform an operation of soldering such a suspension in a narrow space, and defective rates are high. Further, if a current is supplied to the suspension, heat is generated in a soldered portion, causing failure.